While he was running for president, Romney released two years'
worth of tax returns that showed he had paid an effective rate of
less than 15%. His release came after Democrats and the Obama
campaign hounded Romney about his taxes during the 2012
presidential race.

Romney is now calling on all the Republican candidates to do the
same, but he has zeroed in on Trump's supposed hesitance to do
so.

"The reason I think there's a bombshell in there is that every
time he's asked about his taxes, he dodges or delays and says,
'Well, we're working on it,'" Romney said of Trump.

Romney speculated about why Trump might be reluctant to release
his tax returns:

Either he's not anywhere near as wealthy as he says he is, or he
hasn't been paying the kind of taxes we would expect him to pay.
Or perhaps he hasn't been giving money to the vets or the
disabled, like he's been telling us he's been doing.

After Romney's comments on Fox, Trump mocked Romney as a "tough guy" on Twitter.
Trump also recalled his endorsement of Romney in 2012, saying he
"should have known" Romney would lose to President Barack Obama.

"When Mitt Romney asked me for my endorsement last time around,
he was so awkward and goofy that we all should have known he
could not win!" he
tweeted.